This section provides background information related to the present disclosure which is not necessarily prior art.
In view of increased consumer popularity in four-wheel drive vehicles, power transfer systems are currently being utilized in vehicular drivetrain applications for selectively directing power (i.e., drive torque) from the powertrain to all four wheels of the vehicle. In many power transfer systems, a transfer case is incorporated into the drivetrain and is operable in a four-wheel drive mode for delivering drive torque from the powertrain to both the front and rear wheels. Many conventional transfer cases are equipped with a mode shift mechanism that can be selectively actuated to shift between a two-wheel drive mode and a locked four-wheel drive mode. In addition, many transfer cases also include a range shift mechanism which can be selectively actuated by the vehicle operator for shifting between four-wheel high-range and low range drive modes.
It is also known to use “on-demand” power transfer systems for automatically distributing drive torque between the front and rear wheels, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a multi-plate friction clutch assembly and a power-operated clutch actuator that are interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the friction clutch assembly is typically maintained in a released condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch actuator is actuated to engage the friction clutch assembly for transmitting drive torque “on-demand” to the front wheels. Moreover, the amount of drive torque transferred through the friction clutch assembly to the front wheels can be varied as a function of specific vehicle dynamics and operating characteristics, as detected by the sensor arrangement.
A majority of current on-demand transfer cases are configured to include a rear output shaft interconnecting the transmission output to the rear driveline, a front output shaft interconnected to the front driveline, a transfer assembly driven by the front output shaft, and the friction clutch assembly which is operably arranged to couple the transfer assembly to the rear output shaft for transmitting drive torque to the front driveline. Typically, the transfer assembly includes a first sprocket rotatably supported on the rear output shaft, a second sprocket fixed to the front output shaft, and a chain encircling and drivingly interconnecting the first sprocket for common rotation with the front output shaft. The friction clutch assembly and components of the power-operated clutch actuator are disposed to surround the rear output shaft and function to couple the first sprocket to the rear output shaft. Examples of such on-demand or “active” transfer cases are disclosed in U.S. Pat. Nos. 8,091,451; 8,316,738; and 8,678,158.
Such active transfer cases also require a lubrication system for lubricating the clutch assembly and other rotary components mounted on the rear output shaft. A sump of lubricant is maintained in a lower portion of the transfer case so as to typically submerge at least a portion of the second sprocket. A passive lubrication system utilizes lubricant splashed throughout the transfer case upon rotation of the sprockets to lubricate the rotary components and cool the clutch assembly. Examples of transfer cases equipped with passive lubrication systems are shown in U.S. Pat. Nos. 7,753,173 and 8,650,980. As an alternative, a shaft-driven lube pump, such as a gerotor pump, can be operably associated with the rear output shaft to pump lubricant from the sump and distribute the lubricant in response to rotation of the rear output shaft. Finally, it is also known to equip the transfer case with an electric lube pump that can be controlled to provide adaptive lubricant flow.
In the past, the vehicle ride height and suspension configuration of many trucks and sport utility vehicles provided sufficient packaging volume for such traditional active transfer cases. However, in view of increased demand for smaller four-wheel drive vehicles, the packaging volume allocated to the powertrain and the transfer case has been greatly reduced. As such, some transfer cases have been developed which position the friction clutch assembly and the power-operated clutch actuator on the front output shaft. One example of such one-speed on-demand transfer cases can be seen in U.S. Pat. No. 8,157,072.
While such alternative transfer cases address the need for reduced packaging requirements, a need still exists to advance the technology and structure of transfer cases to provide enhanced arrangements that improve upon the prior art.